This invention relates to the fabrication of ferroelectric memory devices, and more particularly to the use of specifically oriented ferroelectric thin films which exhibit both very good ferroelectric properties and lower surface roughness.
The use of ferroelectric thin films in non-volatile memories has drawn much attention in recent years, primarily due to the bi-stable nature of these films, i.e., these films have two predominant polarization directions. Most of the studies of Ferroelectric Random Access Memories (FRAMs) have concentrated on structures having one transistor and one capacitor. The capacitor is made of a thin ferroelectric film sandwiched between two conductive electrodes. The circuit configuration and read/write sequence of this type memory are similar to that of DRAMs, except that data refreshing is not necessary in FRAMs.
Another aspect of ferroelectric non-volatile memory formation is to determine the best technique for deposition of a ferroelectric thin film directly onto the gate area of a FET and to form a ferroelectric-gate controlled FET (FGCFET). Ferroelectric-gate-controlled devices, such as metal-ferroelectric-silicon (MFS) FETs, have been proposed as early as the 1950s, and various modified MFSFET structures have been proposed, for example, metal-ferroelectric-insulator-silicon (MFIS) FETs, metal-ferroelectric-metal-silicon (MFMS) FETs, and metal-ferroelectric-metal-oxide-silicon (MFMOS) FETs.
In order to meet the requirements of FRAMs applications, however, specifically oriented ferroelectric thin films are needed. Experimental results demonstrate that, using metal organic chemical vapor deposition (MOCVD) processes, high oriented, i.e., wherein the ration of c-axis peak-to-random peak is larger than 80%, ferroelectric thin films always have an unacceptable surface roughness because of the growth process using MOCVD.
A method of fabricating a c-axis ferroelectric thin film includes preparing a substrate; depositing a layer of ferroelectric material by metal organic chemical vapor deposition, including using a precursor solution having a ferroelectric material concentration of about 0.1 M/L at a vaporizer temperature of between about 140xc2x0 C. to 200xc2x0 C.; and annealing the substrate and the ferroelectric material at a temperature between about 500xc2x0 C. to 560xc2x0 C. for between about 30 minutes to 120 minutes.
An object of the invention is to provide fabrication processes, such as MOCVD and annealing, for ferroelectric thin films having a preferred orientation.
Another object of the invention is to provide a ferroelectric thin film having minimal surface roughness, uniform surface morphology and improved ferroelectric properties through the use of multiple MOCVD and annealing processes.
This summary and objectives of the invention are provided to enable quick comprehension of the nature of the invention. A more thorough understanding of the invention may be obtained by reference to the following detailed description of the preferred embodiment of the invention in connection with the drawings.